Figure 5.
Active site of the
chymotrypsin, an enzyme that breaks specific peptide
bonds of protein. A
phenylalanine residue (blue ring) of a blue protein
(could be any protein)
slips into the active site of chymotrypsin (green)
at its binding site.
This positions the peptide bond (red) next to
phenylalanine in such a way
that it becomes susceptible to cleavage. It
becomes susceptible because
of a particular confluence of amino acids at
the active site. In brief,
the oxygen of a serine at the 195th position in
the amino acid chain of
chymotrypsin is close to the carboxylate group of
the phenylalanine residue
and can attack the carbon. The carbon is poised
for attack because hydrogens
from both serine-195 and glycine 193 hydrogen
bond with an oxygen from the
carboxylate group, reducing the electron
density around the carbon. At the
same time, the hydrogen that is normally
on the serine oxygen is drawn off
by the electrons on the ring of histidine
at position 57. Those electrons
are more available owing to the interaction
of a hydrogen on the same histidine
ring with the carboxylate group of
aspartate at position 102. After the
peptide bond is cleaved, the two
resulting fragments of the substrate protein
float away, leaving the active
site of chymotrypsin available for a new
substrate.